The present invention relates to flexible wiring harnesses. It particularly relates to wiring harnesses in which flexible sheets are bonded to conductors to form a strong, flexible and corrosion resistant lamination.
In the manufacture of flexible printed wiring harnesses, a number of approaches have been used. A first of these may be called die-cut and glue. By this process a copper conductor is die cut in the desired configuration and is the glued to a plastic backing sheet. This process is generally limited to comparatively small sheets in sizes ranging up to eighteen inches in length.
A second known approach may be called the print and etch method. In this process, copper is laminated to a plastic backing. The copper is covered then with "photoresist." The uncured photoresist is covered with a high contrast negative of the final circuit which is then exposed to ultraviolet light. The light hardens the photoresist under transparent areas of the negative. The still soft photoresist corresponding to the opaque areas of the negative is washed off. The remaining sandwich is flooded (most efficiently by spraying) with a material that dissolves copper but not hardened photoresist or plastic backing. After the unwanted copper has been etched away, the remaining assembly is washed and another material is used to dissolve the remaining photoresist.
This second process involves many steps including the use of a number of chemicals which must be regenerated or replaced frequently. Regeneration involves a lot of chemical equipment while replacement means throwing away all remaining effective material as well as the used up portion. The amount of copper which must be reclaimed or is otherwise wasted is a large proportion of the copper actually remaining in use. Other problems arise, such as undercutting of the remaining copper so that the completed product is frequently unsatisfactory. Finally, the level of labor, skill and interest required by this process is high.
In a third process, a plastic base is printed with a pattern of suitable conductive material. The pattern is then electroplated with copper. The major disadvantage of this technique is the length of time necessary to develop plating of the desired thickness which must approximate 0.010 inch thickness.
Using any of these techniques, the end product produced may have conductors having varied cross-sections resulting in varied resistance characteristics, which is undesirable in many applications.